Let's assume that we have a mechanism for producing EM radiation suspended in the air, and that that mechanism itself is invisible to the naked eye (e.g. a microscopic light bulb on a microscopic wire or a suspended molecular reaction giving off energy.) When off, the light source would not be visible. When turned on however, any light cast from the source to the environment around it would also reach the viewer's eye, and identify the source of light.

Is it theoretically possible to cause the lit light source to cast shadows on the environment that are visible, but for the source itself to remain invisible? As an example, most 3D graphics programs can create invisible light sources which are only identifiable by the scene they illuminate.

I imagine this may be possible with certain environments, such as an environment covered in phosphorescent paint and a black light source, where the light source is not seen but the environment which it illuminates is identifiable.

But what about in the general case, when we limit the variables to the source of the light itself, stipulating it must work in a general environment?

Think about this. A shadow is noticed when the difference in reflected intensity between the places where the source light doses and does not shine is detectible to the eye, and the in ordinary materials albedo is confined to be between 0 and 1. Now ask, is the difference in intensity between look at the source and not looking at the source detectible? Why or why not?
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dmckee♦Mar 9 '13 at 16:52

@Dan Rasmussen Am I Missing something really crucial here? What about a bottle of a gass emitting X-rays and falling on a metallic object, like a key, casting a shadow on a film or paper? Wouldn't that suffice?
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JKLMar 10 '13 at 2:24

1 Answer
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There are multiple ways in which you can in principle disguise the source of light, but photons still must be emitted in order to illuminate something. So a shadow can only really be generated if there is a source of light, although the specific location of the source can be easily masked.

The simplest example is the case of knife-edge diffraction, which is used to send signals over obstructions, particularly in radio wave propagation. Essentially, the transmitter is behind a relatively sharp edge object (like a mountain in the case of radio waves), and the light will actually "bend" around the object to a receiver on the other side.

Much of the research into metamaterials is focused on different applications involving refracting light using materials with negative refractive indexes. These materials play key roles in the concepts of modern "cloaking devices" where the idea there is to mask an object in a cocoon of meta material well into the visible spectrum.

If you get into the metamaterial realm, the idea might be that one could not only bend the light around the object, but amplify the light in some way so that in some arbitrary direction light passing through the metamaterial exits with greater intensity than the background ambient light such that an object in the path of the amplified light would cast a shadow. I would imagine that the impression of the observer would be that some region they are looking at somehow appear brighter, although the exact location of the light source might not be identified exactly.

So in general it is possible to mask the specific location of a source, but you simply shouldn't be able to generate a shadow without there being some change in intensity between the front and back of the object casting the shadow.